Cellulosic and lignocellulosic feedstocks and wastes, such as agricultural residues, wood, forestry wastes, sludge from paper manufacture, and municipal and industrial solid wastes, provide a potentially large renewable feedstock for the production of valuable products such as fuels and other chemicals. Cellulosic and lignocellulosic feedstocks and wastes, composed of carbohydrate polymers comprising cellulose, hemicellulose, and lignin are generally treated by a variety of chemical, mechanical and enzymatic means to release primarily hexose and pentose sugars, which can then be fermented to useful products.
Pretreatment methods are used to make the carbohydrate polymers of cellulosic and lignocellulosic materials more readily available to saccharification enzymes. Standard pretreatment methods have historically utilized primarily strong acids at high temperatures; however due to high energy costs, high equipment costs, high pretreatment catalyst recovery costs and incompatibility with saccharification enzymes, alternative methods are being developed, such as enzymatic pretreatment, or the use of acid or base at milder temperatures where decreased hydrolysis of biomass carbohydrate polymers occurs during pretreatment, requiring improved enzyme systems to saccharify both cellulose and hemicellulose.
Teixeira, L., et al. (Appl. Biochem. and Biotech. (1999) 77-79:19-34) disclosed a series of biomass pretreatments using stoichiometric amounts of sodium hydroxide and ammonium hydroxide, with very low biomass concentration. The ratio of solution to biomass is 14:1.
Elshafei, A. et al. (Bioresource Tech. (1991) 35:73-80) examined the pretreatment of corn stover utilizing NaOH. Kim, T. and Y. Lee (Bioresource Technology (2005) 96:2007-2013) report the use of high amounts of aqueous ammonia for the pretreatment of corn stover.
Int'l. Pat. App. Pub. No. WO2004/081185 discusses methods for hydrolyzing lignocellulose, comprising contacting the lignocellulose with a chemical; the chemical may be a base, such as sodium carbonate or potassium hydroxide, at a pH of about 9 to about 14, under moderate conditions of temperature, pressure and pH.
U.S. Pat. Nos. 5,916,780 and 6,090,595, describe a pretreatment process wherein a specified ratio of arabinoxylan to total nonstarch polysaccharides (AX/NSP) is assessed and used to select the feedstock.
U.S. Pat. No. 7,354,743 discloses methods for degrading a lignocellulosic material, comprising treating the lignocellulosic material with an effective amount of one or more cellulolytic enzymes in the presence of at least one surfactant; the presence of the surfactant increases the degradation of lignocellulosic material compared to the absence of the surfactant.
Borjesson, J. et al. (Enzyme and Microbial. Technology (2007) 40:754-762) focused on the enzymatic hydrolysis of the softwood substrate spruce lignocellulose and aimed to give further understanding of the mechanism behind the enhancing effect on the conversion by addition of ethylene oxide based surfactants and polymers. No effect of PEG was seen on a delignified substrate.
Int'l. Pat. App. Pub. No. W02008134037 added surfactants in the pretreatment step to enhance the removal of lignin in corn stover biomass in an effort to increase the digestibility of the delignified biomass. However, not considered was a reduction in enzyme loading nor reported was a saccharification product comprising at least about 7 percent by weight sugars in a 24 hour period after contact with an enzyme consortium.
Most pretreatments such as the ones described above either result in a pretreated biomass depleted of lignin and hemicellulose or the partial depletion of hemicellulose with retention of most of the lignin. Therefore a method is needed to selectively remove only lignin without significant loss of either hemicellulose or cellulose from the biomass, as these constitute the source of sugars for fermentation. Thus, none of these references relates to the unpredicted mechanism recited herein: retaining hemicellulose or cellulose in the biomass such that the saccharification product comprises at least about 7 percent by weight sugars in a 24 hour period after contact with an enzyme consortium.
In order to be economically competitive, a commercial process for the production of sugars from a renewable resource biomass requires the hydrolysis of carbohydrates in lignocellulosic biomass to provide high yields of sugars at high concentrations using low amounts of chemicals.